The present invention pertains to techniques for processing data signals. More particularly, the present invention relates to methods and apparatus for filtering unwanted noise from data-carrying signals, and finds particular application to the filtering out of periodic noise, and especially periodic noise with harmonics. Such periodic noise with harmonics may be present in data signals obtained from downhole instrumentation in drilling processes due to the operation of fluid pumps utilized in circulating drilling fluid and/or the operation of power units such as rotary tables and the like utilized to provide rotational movement to drill bits.
Some filter constructions are known for removing periodic noise with harmonics from a signal. Generally, such a filter would provide stopbands at integer multiples of a base frequency throughout at least a portion of the frequency response of the filter. For example, a comb-type filter provides such a frequency response. A bucket brigade filter is an example of such a comb-type filter. In a bucket brigade, the input signal is combined in a difference amplifier with the same signal delayed, ideally for an amount of time equal to the base period of the noise to be eliminated. Thus, the noise portion of the signal is subtracted from the same signal one period later to eliminate the periodic noise. If the delay mechanism is set to equal the exact period of the noise to be eliminated, the base frequency of the noise and all the harmonics thereof would be eliminated, providing the amplitude of the noise components does not vary with time. However, if the period of the noise to be eliminated drifts with time, the effectiveness of the filter may be greatly reduced for the noise would not be entirely eliminated.
Generally, the time delay mechanism may be varied within limits by an operator, for example, making necessary adjustments to the hard wire delay mechanism or, if the filter is provided by means of a computer, by providing appropriate instructions.
However, such operator adjustments are not responsive to slight, fairly quick noise frequency drifts which may nevertheless affect the filter output.
An additional problem may be introduced by utilizing a bucket brigade filter. The signal passing through the delay mechanism and combining with the original signal in the difference amplifier will introduce ghosts of non-periodic signal components, whether such components are data-carrying information components or other types of noise. Further, the delay mechanism can possibly be set to cause total or partial overlap of a data component passing through the delay mechanism with a subsequent data component communicated directly to the differencer. Consequently, the output of the filter may not only be further cluttered with ghosts of non-periodic signal components, but may also be characterized by distorted or even lost information-bearing signal components.
A Costas type filter may also be utilized to selectively filter harmonics of a periodic noise.
For the effective and efficient elimination of periodic noise, and particularly periodic noise featuring harmonics, it is desirable and advantageous to provide a filtering technique featuring a frequency response with stopbands arranged at integer multiples of a selectable base frequency which is automatically adapted to the possible changing period of the noise to be eliminated, wherein any ghost signals that may be generated by operation of the filter are rapidly attenuated to insignificant levels, and wherein distortion of data signal components is minimized.